1. Field of the Invention
The present invention relates generally to a seam welding machine and a method of protecting welding electrodes of the same from degradation. More particularly, the invention is concerned with a seam welding machine of a structure which is capable of protecting peripheral surfaces of welding electrodes or surfaces of welded product against deposition of an agent used for surface treatment of sheet-like materials to be welded together, by applying an electrode protecting agent onto peripheral surfaces of the welding electrodes. The invention is also directed to a method of protecting the welding electrodes of such a seam welding machine.
2. Description of Related Art
For having a better understanding of the present invention, description will first be directed to the background techniques thereof. FIGS. 7A to 7C are schematic views for illustrating a conventional welding method carried out by a seam welding machine known heretofore, which is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 258382/1992 (JP-A-4-258382). Referring to the figures, there are mounted on a common base (not shown) of the welding machine a clamping device 4 positioned at an exit side and a clamping device 5 positioned at an entrance side in such disposition that respective tip ends of both the clamping devices 4 and 5 face in opposition to each other. Strips 1a and 2a having respective cut end portions along which the strips 1a and 2a (i.e., materials to be welded) are to be welded together are fixedly held by the clamping devices 4 and 5, respectively, in a sandwiched manner. Upon welding, the clamping device 4 located at the exit side is slightly swung upwardly, while the clamping device 5 of the entrance side is caused to move forwardly in the feeding direction of the strip 2a as indicated by arrows A1 and A2, respectively, in FIG. 7A.
Subsequently, the cut end portions of the strips 1a and 2a are superposed onto each other over a width sufficient for welding, as can be seen from FIG. 7B. The superposed end portions of the strips 1a and 2a are pressed against each other in a vertical direction by means of a pair of upper and lower welding electrodes 8a and 8b each of a rotatable disk type fixedly mounted on a movable carriage (not shown), which is moved in a direction orthogonal to the direction in which the strips 1a and 2a during the welding operation are fed, as indicated by an arrow A3 in FIG. 7C. In this state, a welding current is applied across the upper and lower welding electrodes 8a and 8b from a DC power supply apparatus (not shown), as a result of which a welding current flows through the superposed end portions of the strips 1a and 2a. In this manner, the seam welding is carried out.
The conventional seam welding machine briefly described above suffers a problem that when strips such as steel sheets which have previously undergone a surface treatment, such as plating are to be welded together, the plated material tends to be deposited on the electrode surfaces and/or the welded product. Additionally, the materials forming surface portions of the welding electrodes are likely to alloy with the plated material, involving degradation as well as nonuniformity in the quality of welding.
More specifically, because a melting point of a plated material is normally lower than that of the steel sheet, i.e., the material which will undergo the welding, the plated material can easily be melted by the heat of the welding to be alloyed with the surface material of the welding electrodes or deposited as oxides on the electrode surfaces. As a consequence, electrical conductivity as well as thermal conductivity of the surface portions of the welding electrodes is impaired, incurring such unwanted situation that the heat input balance which is one of the most important factors for resistance welding is lost (i.e., the heat input to a combination of the welding electrodes and the materials to be welded increases when compared with the heat input to the materials to be welded), which eventually involves deterioration in the quality of welding.
For coping with the problems mentioned above, there has already been proposed a method of protecting the welding electrodes from the unwanted phenomenon, as is described, for example, in Japanese Unexamined Patent Application Publication No. 24239/1979 (JP-A-54-24239), according to which an electrode protecting agent is sprayed onto tip or peripheral end portions of the welding electrodes by using nozzles to thereby protect the surface of the welding electrodes from contamination with the plated material. However, in the welding electrode protecting methods known heretofore, no consideration is paid to the post-treatment of the electrode protecting agent as applied to the welding electrodes. Additionally, it is noted that no measures are proposed for controlling appropriately or properly the amount of the electrode protecting agent as applied by the spraying.
Such being the circumstances, when no post-treatment is performed after spraying the electrode protecting agent or when an unnecessarily large amount of the electrode protecting agent is deposited on the electrode surface, the electrode protecting agent is prone to drop onto a surface of strips as welded. In this conjunction, it should be mentioned that when the state in which an oily electrode protecting agent is deposited on the surface of the strip materials as welded is left as it is, finishing treatments such as color coating of the welded strip is likely to suffer a difficulty such as a fault in the coating or the like, causing the outer appearance of the finished surface of a steel sheet manufactured as a final product to be inattractive. For this reason, it is extremely important to prevent the electrode protecting agent from being deposited in an unnecessarily large amount. Of course, situations in which the electrode protecting agent remains as deposited on the surface of the strip or sheet which has undergone welding has to be avoided without fail.